Question

Find the power of a concave lens of focal length 2 m.

Answer 1

Step1: Given data

Focal length ( $f$ ) = $2m$

Step2: Concave lens

1. This type of lens diverges a straight beam of light from the source to an upright and virtual image called a concave lens.
2. This type of lens can form both types of images real as well as virtual images.
3. According to the sign convention, the focal length of a concave lens is always negative.

Step3: Calculating the power of the concave lens

The power of the lens can be calculated as

$P=\frac{1}{f}[where\text{'}P;ispowerand\text{'}f\text{'}isfocallength]$

Putting the value of $f$

According to the sign convention, the focal length of a concave lens is always negative.

$$\begin{gathered} P=\frac{1}{-2} \\ P=-0.5D \end{gathered}$$

Hence, the power of a concave lens of focal length $2m$ is $-0.5D$